With the increase of communication traffic in a backbone optical network, the capacity of an optical communication system has been increased. As an optical transmitter installed on an optical network, a Mach-Zehnder (MZ) type optical modulator capable of modulating light at a high speed is used. Further, in order to optimize the characteristic of an optical signal transmitted by the MZ type optical modulator, for example, the Mach-Zehnder type optical modulator has an automatic bias control (ABC) circuit that prevents a transmission signal from being deteriorated by drift. The ABC circuit applies each bias voltage (hereinafter, referred to as a “bias” in some cases) of I, Q, and φ to the modulator.
Since the operation characteristics of the ABC circuit drift by a variation of a temperature or aging, the bias may not be operated with a fixed value. As a result, in order to maintain the quality of the optical signal, it is necessary to constantly feedback-control the output fluctuation of the modulator at all times and make the bias to follow an optimum point.
As a feedback control method of the ABC circuit, there is proposed a method that adjusts a bias voltage by superimposing a low-frequency pilot signal (pilot tone) with respect to the branch outputs of the optical modulator so as to minimize a monitor amplitude of the pilot tone (see, e.g., Japanese Laid-Open Patent Publication No. 2008-092172).
A Dual-Polarization (DP)-N-Quadrature Amplitude Modulation (QAM) MZ type optical modulator (N represents the number of symbols) bias-controls 6 points of, for example, I, Q, and φ at an X side and I, Q, and φ at a Y side. The biases of I, Q, and φ may be similarly controlled by the pilot tone superimposing method.
FIG. 10 is a diagram for describing a pull-in point of the φ bias. The horizontal axis represents the φ bias, and the vertical axis represents a control error amount of the pilot tone superimposing method. As the control error amount at the time of executing an ABC, 2f0 is detected in synchronization detection of the ABC circuit during an optimal control when the pilot tone has a frequency of f0. When the control error amount deviates from the optimal point, f0 is detected. As illustrated in FIG. 10, when the φ bias is changed, an accurately controlled position (the error amount is 0) exists every 180° based on a point of 90° (270°, 450°=90°).
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2008-092172.